In the drilling of oil and gas wells, drilling mud is circulated down the interior of a hollow drill string, through nozzles in a drill bit located at the bottom of the drill string, and back up to the surface through the space between the drill string and the wall of the wellbore. The drilling mud is typically either water-base or oil-base and contains a variety of components. The primary functions of the drilling mud are to lubricate the drill bit, to transport rock cuttings to the surface and to maintain a hydrostatic pressure in the wellbore sufficient to prevent the intrusion of formation fluids and thereby prevent blowouts.
Following drilling, casing is cemented in the wellbore to prevent caving in of the hole and to segregate the formations penetrated. A casing string is lowered into the wellbore and a cement slurry is pumped into the annulus between the casing and the wall of the wellbore. This is usually accomplished by pumping the cement slurry downward through the interior of the casing and upward through the annulus. Once the cement slurry has filled the annular interval to be cemented, the pumping is stopped and the cement is allowed to set.
It is important for the cement to form a strong, continuous sheath which bonds the casing to the wall of the wellbore. The cement should completely surround the circumference of the casing and should extend uniformly through the vertical length of the annular interval cemented. If the cement is weak, or if any voids are left therein, several undesirable consequences can result. A poor cementing job will not effectively segregate the formations penetrated by the wellbore, and unwanted communication between the formations may occur, sometimes resulting in the production of unwanted fluids. Also, production fluid from a petroleum bearing formation may flow through channels in the cement and into another formation, where it is lost. This is especially disadvantageous when the other formation contains an aquifer. Contamination of the petroleum bearing formation itself can also occur, such as when salt water PG,4 channels through the cement and flows into the petroleum bearing formation. Another deleterious consequence of a poor cementing job can be the loss of treatment fluids which are pumped down the well to stimulate production.
One of the most common causes of ineffective cementing jobs is the failure to displace all of the drilling mud and filter cake from the annulus prior to introduction of the cement slurry. Filter cake is a layer of solids concentrated from the drilling mud, which most commonly builds up on the wall of the wellbore opposite permeable formations. Even relatively small amounts of drilling mud and filter cake can contaminate the cement slurry and cause weak spots in the cement. Large quantities can obstruct the flow of the cement slurry and thus prevent the cement slurry from completely surrounding the casing, thereby resulting in channels through the cement.
A great deal of effort has gone into developing methods and apparatus for effectively removing drilling mud and filter cake from the annulus so that the cement slurry will not be obstructed or contaminated. Numerous preflush fluids have been developed, some with thickening agents, which are pumped through the annulus ahead of the cement slurry in an attempt to flush drilling mud and filter cake out of the annulus. Alteration of the properties of the cement slurry itself has also been tested. One common practice is to use centralizers and scratchers on the casing to scrape filter cake from the wall of the wellbore as the casing string is lowered into place. Although these methods and apparatus having provided some benefit, ineffective cementing jobs caused by incomplete displacement of drilling mud and filter cake are still common. When this occurs, expensive remedial cementing is often required, which carries with it the additional cost of the revenues lost while the well is shut in for the remedial work.
There still exists a great need in the art for a method of cementing wells which will prevent drilling mud and filter cake from causing weak spots and channels in the cement. The present invention is aimed at providing such a method.